1. Field of the Invention
The present invention relates to a method and apparatus for the continuous treatments of iron members or parts wherein a furnace brazing procedure and a gas soft-nitriding procedure are combined such that the iron article is successively treated by such combined procedures.
2. Description of Relevant Art
A furnace brazing technique is known as a method of soldering machine parts and is suitable for mass-producing soldered parts of stable and uniform quality. In such known furnace brazing method, a protecting controlled gas is introduced into the furnace to prevent the parts to be treated, as well as a brazing material, from being oxidized or decarbonized. The brazing material is heated to a temperature higher than its melting point to conduct the brazing treatment. Such controlled gases include, for example, endothermic gases (RX gases), exothermic gases (NX and DX gases), and the like. The brazing temperature varies depending on the type of brazing material, e.g., it is generally in the range of 1090.degree. C. to 1150.degree. C. for brazing copper, and 910.degree. C. to 980.degree. C. for brazing silver. Illustrative of known brazing techniques are those disclosed in U.S. Pat. No. 2,908,072 entitled "BRAZING PASTE AND PROCESS OF BRAZING" and U.S. Pat. No. 3,055,097 entitled "METHOD OF MAKING A REINFORCED BRAZED TUBULAR JOINT".
In order to improve fatigue and wear resistance of iron members, there have been proposed and practiced gas soft-nitriding methods wherein an iron member is treated to form a hard nitriding layer on the surface portion thereof. As is well known, the gas soft-nitriding method employs a mixed gas of a carrier gas such as the above-mentioned endothermic gas (RX gas) or exothermic gas (NX gas), to which is added ammonia gas (NH.sub.3), and the gas soft-nitriding treatment is carried out at 550.degree.-620.degree. C. in such atmosphere. By such treatment, the iron member is provided in the surface portion thereof with a crystalline layer called an ".epsilon. phase", composed of iron, nitrogen and carbon (Fe-N-C), serving to enhance fatigue and wear resistance, as well as to enhance the mechanical strength of the member.
Such brazing and gas soft-nitriding treatments have heretofore been conducted separately for soldering metal parts by the brazing and for improving the mechanical strength of the parts by the gas soft-nitriding treatment. It is common practice that when improved mechanical strength is necessary, the brazing treatment is first conducted as required and then the brazed parts are subjected to the gas soft-nitriding treatment in a separate step or line, such as disclosed at page 599 (Example 545) of the Metals Handbook, 8th Edition, Volume 6 entitled "Welding and Brazing" (by the American Society for Metals). Thus, according to known treating procedures, parts of lowcarbon steel or other steels of low quality are first brazed and then the brazed parts must be subjected to a gas soft-nitriding treatment or other treatments, when it is required to improve the mechanical strength thereof, whereupon the brazed parts are again heated and treated in a completely separate apparatus, step, or line.
In such known methods, when the successive procedures of brazing and gas soft-nitriding treatments are required, there arises a number of problems. For example, an increased number of steps and additional equipment are required, poor treating and working efficiency results because the treatments are effected in separate lines, the cost of equipment is increased, additional work is required because of the separation in procedures, and additional space for installation of lines is also required. Such attendant problems render the methods unfavorable for mass-production of parts which are to be brazed and subsequently gas soft-nitrided, inevitably resulting in an increased cost of the final products. In addition, the known separate procedures are not favorable due to a loss of heat energy because separate heating treatments are required for the brazing and gas soft-nitriding procedures, respectively. Further, the re-heating of brazed parts may undesirably involve thermal stress of the product, thus lowering the quality of the product, and leading to a lowering of yield of the product.
The present invention effectively solves the above-discussed problems encountered in the known methods wherein the brazing and gas soft-nitriding procedures are carried out separately. In the present invention, the brazing procedure is conducted under heating conditions wherein heat energy is able to be used for subsequent treatments, and the controlled gas for the brazing has substantially the same standard composition as a carrier gas for the gas soft-nitriding treatment.